Field of the Invention
The present invention relates to a gas sensor composed with an oxygen ion conductive solid electrolyte, and particularly relates to a NOx sensor.
Description of the Background Art
Conventionally, various measuring devices have been used for determining a concentration of a desired gas component in a measurement gas. A known device of measuring a NOx concentration in a measurement gas such as a combustion gas, for example, employs a Nox sensor formed with an oxygen ion conductive solid electrolyte, such as zirconia (ZrO2) (see Japanese Patent Application Laid-Open No. 8-271476, for example).
According to a Nox sensor disclosed in Japanese Patent Application Laid-Open No. 8-271476, O2 in a measurement gas introduced from outside is previously removed by pumping in the first internal space so as to make the measurement gas to be in a state of low oxygen partial pressure (a state in which oxygen partial pressure is lowered to the extent that measurement of NOx is not affected by the presence of O2 in the measurement gas), and then, the measurement gas is introduced into the second internal space. NOx is reduced at the measuring electrode by applying a constant voltage between the measuring electrode containing such as Pt and Rh provided in the second internal space and the reference electrode provided in the air. The NOx concentration is to be detected based on the current value of the current flowing at that time between the measuring electrode and the reference electrode, the current value being proportional to the NOx concentration.
The heater for electrically heating the NOx sensor is provided inside the NOx sensor. When detecting NOx in the aforementioned manner, heating is performed by the heater such that temperatures of the first and second internal spaces (and temperatures of electrodes provided for each internal space) satisfy the predetermined condition. In addition, the heating also helps to enhance oxygen ion conductivity of solid electrolyte composing the NOx sensor.
According to the Nox sensor disclosed in Japanese Patent Application Laid-Open No. 8-271476, the heating temperature of the heater is controlled on the basis of electric resistance value (resistance value) of the heater when applying current. Accordingly, it is necessary to accurately obtain the resistance value of the heater to control the heating temperature with precision.
The resistance value can be obtained by subtracting the resistance values of two leads (heater leads) connected to the both ends of the heater from the entire resistance value combining the heater and the two heater leads. According to the above, if two heater leads are provided in a manner that the resistance values are equal to each other, the resistance value of the heater can be obtained by subtracting the doubled value of the resistance value of one of the heater leads. In this case, it is necessary to ensure that the resistance values of the two heater leads are equal to each other in order to accurately obtain the accurate resistance value of the heater.